The present invention relates to a high frequency heating apparatus, and more particularly to a microwave oven equipped with an electric heating arrangement for ordinary heating by an electric heater as well as high frequency heating of an object to be heated.
Commonly, in microwave ovens, high frequency oscillator tubes such as magnetrons are generally employed for subjecting food materials which have been placed in a heating cavity or heating chamber to be cooked for a high frequency heating or dielectric heating, while some types of such microwave ovens are further equipped with electric heating arrangements to heat the materials to be cooked in one heating cavity through heating by an ordinary electric heater as well as heating by high frequency waves normally in microwave ranges either simultaneously or independently for formation of surface browning or scorching of the food materials to such an extent as will stimulate one's appetite. Particularly, microwave ovens of the latter type have come into wide use because of their useful features as cooking apparatus that the high cooking speed of microwave ovens is advantageously combined with the ability to form the desirable surface browning on the food materials to be cooked.
Conventionally, the electric heating arrangements incorporated in the known microwave ovens of the above described latter kind are broadly divided into two types, i.e., one type disclosed, for example, in U.S. Pat. No. 3,081,392 and employing a so called "sheathed heater" prepared by enclosing a resistant wire for electrical heating in a metal sheath through insulating material, and the other type proposed, for example, by U.S. Pat. No. 3,196,243 and including an unshielded or exposed infrared heater element. Of these two types, the former has been widely adopted, mainly because of its simple and rigid construction, and high heating efficiency.
While the known microwave ovens of the above described type which employ the sheathed heater for effecting the ordinary electric heating as well as the high frequency heating in one heating cavity have the advantage as described earlier, they have problems related to cleaning of the interior of the heating cavity. More specifically, in the heating cavity normally surrounded by an oven defining structure preferably of double wall construction and having a rectangular cubic space defined by six walls, cleaning of at least one of the six walls is made extremely difficult due to provision of the heater element for the electric heating arrangement on such particular one of the cavity defining walls. In other words, while oil drops, juice, crumbs and the like produced by the food materials during cooking and adhering to the cavity walls tend to soil inner surfaces of all the walls forming the heating cavity, such soiling usually becomes very difficult to remove due to heating and drying by the electric heating device especially at the portion of the particular one wall behind the electric heater element.
Although drawbacks as described above may be overcome by making such heater element readily attachable to or detachable from the particular wall, no countermeasures sufficiently effective for the purpose have been proposed in the prior art arrangement described earlier. More specifically, in U.S. Pat. No. 3,196,243 referred to earlier, several fixing screws are employed as means for securing the heater element to the interior of the heating cavity, which arrangement, however, is not only inconvenient for attaching or detaching such heater element, requiring a particular tool for the purpose, but is accompanied by the possibility of danger in that leakage of microwave energy from a power supplying portion of such heater element tends to be increased due to insufficient securing of the heater element to the interior of the heating cavity arising from wearing out or damage to the securing means through repeated detaching and attaching of such heater element, and thus, an electric heating arrangement having a heater element readily detachable and attachable with negligible high frequency leakage, and fully guaranteed for safety even in case of such high frequency leakage is strongly desired in the art.